Conventionally, the commonest method in health management concerning obesity or other body conditions is to measure the body weight. Nowadays, obesity is not regarded simply as a body type, but people are looking at indices for measuring obesity. One such index is the body fat mass, which indicates the mass of subcutaneous fat and/or visceral fat; another is the body-fat ratio, which indicates the ratio of body fat to body weight.
For many years, measurement of body fat was complex and time consuming. The subject had to be completely submerged under water to measure the underwater body weight and to calculate the specific gravity. On the other hand, however, new and simpler measurement methods are used, which enable us to estimate a result that has a close correlation with the result obtained by conventional methods. Some of these methods use ultrasonic waves, some others use near infrared, and the most common methods use the bioelectrical impedance.
Many bioelectrical impedance measurement methods widely used are based on the measurement principle called the “four-electrode method”. Referring to FIG. 27, a bioelectrical impedance measurement method based on the four-electrode method is described below.
According to this method, a pair of current-carrying electrodes 1a, 1b and a pair of measuring electrodes 2a, 2b are attached to the surface of a living body 5 so that the measuring electrodes 2a, 2b are placed between the current-carrying electrodes 1a, 1b. The current-carrying electrodes 1a, 1b are connected to a radio frequency current source 3 to supply constant radio frequency current i through the living body 5. The voltage (potential difference) produced thereby is measured by a detector 4 connected to the measuring electrodes 2a, 2b. If the inside of the living body 5 is homogeneous, it can be assumed that the current is almost evenly distributed at positions distant from the current-carrying electrodes, 1b. Therefore, by placing the measuring electrodes 2a, 2b away from the current-carrying electrodes 1a, 1b to a predetermined extent, it is possible to obtain the bioelectrical impedance of a part 5a horizontally extending deep in the living body. Also, by setting the input impedance of the detector 4 adequately higher than the impedance of the measuring electrodes 2a, 2b, the influence of the impedance of the electrodes can be eliminated, so that the measurement can be performed accurately.
The bioelectrical impedance measurement method using the four electrodes is applied to various kinds of body composition measurement apparatuses described in documents and/or available on the market. For instance, the Japanese Unexamined Patent Publication No. H7-51242 discloses an apparatus having a pair of grips to be held by the hands, each grip provided with a current-carrying electrode and a measuring electrode. The electrodes are located so that, when the subject holds the grips with the hands, the current-carrying electrode contacts the hand at a part close to the fingers and the measuring electrode contacts the hand at a part dose to the wrist. Then, based on the bioelectrical impedance measured there by, the apparatus estimates various kinds of information about the living body, such as total internal body fat, lean body mass, body-fat ratio, total body water or basal metabolism rate. Also, the Japanese Examined Patent Publication No. H5-49050 discloses an apparatus constructed so that the electrodes contact the bottoms of the feet when the subject puts the feet on it. This apparatus can simultaneously measure the weight and the body fat.
The four-electrode method is widely used because, despite its easiness, it provides high accuracy in the measurement of bioelectrical impedance. As is clear from FIG. 27, however, the method has such a theoretical limit that it can measure the bioelectrical impedance of only the parallel constitution of a large area inside the living body 5 between the two measuring electrodes 2a, 2b, that is, the internal panniculus extending parallel to the surface of the living body 5. Therefore, it is essentially impossible to obtain information about the living body in the depth direction (i.e. the cross-sectional direction of the internal panniculus) below the surface, such as the local thickness of the subcutaneous fat or the thickness of the visceral fat.
By the body composition measurement apparatus using the above-described bioelectrical impedance measurement according to the four-electrode method, the bioelectrical impedance is measured along the current path between the two hands, between the two feet or between one hand and one foot, to estimate the lean body mass. Further, the body fat mass is also estimated using supplementary data relating to body-build, such as the height or the weight of the subject. By this apparatus, a part of the current path goes through the abdomen, whose cross-sectional area is a good deal larger than that of a leg or an arm. This means that the leg or the arm makes a relatively large contribution to the bioelectrical impedance, while the abdominal subcutaneous fat and the intra-peritoneal fat (visceral fat) make a relatively small contribution. Thus, the measurement result poorly reflects an increase or decrease of the abdominal subcutaneous fat or the intra-peritoneal fat, so that the result is not reliable.
Even without measuring the bioelectrical impedance, the information about the living body in the depth direction below the surface of the living body can be obtained with other apparatuses, such as X-ray computed tomography (CT) scanner or magnetic resonance imaging (MRI) apparatus. These apparatuses, however, are very large and very expensive, and they immobilize the subject for a long time. Thus, the subject is forced to suffer physically, mentally and economically. Further, they cannot be used by ordinary people in everyday life for health management or health maintenance.
The present invention has been achieved in view of the above problems, and its main object is to propose a bioelectrical impedance measurement method and apparatus, which give the subject same or less burden as that by the currently practiced measurements of bioelectrical impedance according to the four-electrode method, and which can measure the bioelectrical impedance in the depth direction inside of the living body, which has been difficult to measure by conventional methods.
Another object of the present invention is to propose a measurement apparatus, which uses the result of the measurement of the bioelectrical impedance in the depth direction inside the living body to obtain such information about the living body that is difficult to measure by conventional methods, and to present that information to the subject or to the examiner. Examples of the aforementioned information are the thickness of subcutaneous fat, the steatosis (or fatty change) of muscular tissue, the strength or thickness of osseous tissue (such as cortical bone density or cancellous bone density), and the steatosis of bone marrow cells inside the bone.
Still another object of the present invention is to improve the accuracy of the body composition measurement apparatuses using the conventional four-electrode method, and to propose a health management guideline advising apparatus, which can provide very useful information for guiding health management.